Silicon carbide (SiC) is a well known hard material, with a low coefficient of thermal expansion and inert to a variety of environments such as high temperature oxidation and corrosion by acids. Coatings of dense SiC have been applied to materials such as graphite, silicon or ceramic materials to protect them from oxidation and erosion. The preferred method to produce these coatings is a chemical vapor deposition (CVD) method using methyltrichlorosilane and hydrogen at temperatures between 1000.degree.-1400.degree. C. In this process, the coating is produced primarily by a gas phase reaction. Because of the high temperatures required for this process. It can only be applied to substrates such as graphite, cemented carbide and silicon. While coating of SiC would be very desirable on metallic articles because it would result in good surface properties regarding erosion, corrosion and oxidation as well as good mechanical properties of parts to withstand stress, the high temperatures required would degrade the mechanical properties of the metal. Additionally, when treating metals by this process, there is a problem in the adhesion of the silicon carbide to the metal due to a mismatch in the physical properties between the metal substrate and the SiC ceramic coating and, therefore, the use of metallic interlayers is required.
A second method, glow discharged CVD, is a similar process to CVD but the reaction temperature is lowered by the activation of the gaseous reactants by electrical discharges. Production of SiC coatings on different types of substrates have been obtained by glow discharge activation of silicon halides/hydrocarbon mixtures, such as SiCl.sub.4 or SiH.sub.4 and CH.sub.4 or C.sub.2 H.sub.2. Although the temperature of the reaction can be lowered to about 300.degree. C. the process must be operated at sub-atmospheric pressures. In fact, in order to assure a glow discharge in the gas mixture, the reaction chamber remains under partial vacuum during the deposition step. Another disadvantage of this method is the restriction of substrates with simple shapes in order to assure homogenity of the coating.
Several processes have been attempted in the past to apply SiC coatings to various substrates. Japanese patent application, Sho57-155365 entitled "Method for Preparing a Silicon Carbide Coating With Good Adhesion Properties Over a Metal Substrate Surface", teaches the formation of SiC on Ti, Al or 304 stainless steel substrates by glow discharged CVD. The coating is formed in a mixture of SiH.sub.4 /C.sub.2 H.sub.2 in a 1:2 ratio, at a temperature of 300.degree. C. and a total pressure of 0.3 torr.
Japanese patent application Sho58-22375 entitled "Metallic Material Having an Ultrahard Coating and Method for its Manufacturer" teaches the formation of SiC coatings on substrates consisting primarily of carbon-containing iron by a CVD method. The coating is formed from a mixture of methyltrichlorosilane and H.sub.2 at 1200.degree. C. and at a pressure of 180 torr. Before applying the SiC coating, the substrate is coated with nickel or cobalt as an intermediate layer.
H. E. Hintermann in an article entitled "Tribological and Protective Coating by Chemical Vapor Deposition", Thin Solid Films, 84 (1981) 215-243, teaches the application of refractory coatings for steels and nickel-based alloys. The preferred steels used as substrates are tough hard chromium-containing steels with Mo, V and W added. The preferred method of coating is CVD performed at temperatures between 800.degree.-1000.degree. C. using metal halides.
A paper by F. Bozso, et al. J. Appl. Phys. 57(8), p. 2771 (1985) entitled "Studies of SiC Formation on Si(100) by Chemical Vapor Deposition" describes the formation of SiC by the reaction of a single crystal of silicon (Si(100)) and a molecular beam of C.sub.2 H.sub.4 under ultra high vacuum conditions and at temperatures in excess of 700.degree. C.
An article by G. Verspuri, entitled "CVD of Silicon Carbides and Silicon Nitride on Tools for Electrochemical Machining", Proc. Electrochem. Soc. (1979), Vol. 79-3, describes the formation of SiC on tools made of tungsten and molybdenum by a CVD method at atmospheric pressure using a mixture of dimethyldichlorosilane and H.sub.2 at a temperature of 1300.degree. C.